He also gave some detail on what the first hours of a war would involve.

"The only way to 'locate and destroy - with complete certainty - all components of North Korea's nuclear weapons programs' is through a ground invasion," he wrote in response to Congressman Lieu's questions about a potential conflict.

The risks involved included a potential nuclear counter-attack by North Korea while US forces attempted to disable its "deeply buried, underground facilities", he said.

"A classified briefing is the best venue for a detailed discussion," he added.

The Joint Chiefs of Staff directly advise the president of the United States on military matters.

In a statement with more than a dozen other military veterans turned congressmen, Mr Lieu, a Democrat, said the assessment was "deeply disturbing" and warned that a conflict "could result in hundreds of thousands, or even millions of deaths in just the first few days of fighting."

"Their assessment underscores what we've known all along: there are no good military options for North Korea," the statement said.

The letter was published as Donald Trump begins his mammoth tour of Asia, during which the North Korean threat is expected to be a major topic of discussion.

The president has previously said that if forced to defend the US or its allies, he "will have no choice but to totally destroy North Korea."

I've been following the president's trip to Asia and can confidently say this. We're seeing a replay of the events that led up to Gulf War 1. If you remember we saw last minute diplomacy and heard talk of "breakthroughs" that would prevent war.

In the end it didn't work and war were declared.

I see the same happening here. The weird thing? Early reports had the Pentagon pushing for limited strikes and no boots on the ground outside of a batshit crazy plan for a special ops decapitation raid, not a full scale invasion as this report is laying out.

So if I believe this news is credible (and I do) how does that jive with what we've already read?

Simple.

The Pentagon is simply justifying its rationale, and war planning. They're simply adding a caveat to say that with the plan as it is they can't guarantee that all nuclear weapons will be destroyed.

They're playing politician and telling policy makers that even after this military action, the North could still have nukes.

This news is being put out as a way to lower expectations once the balloon goes up. It's a way to chill out those people that watch military affairs and wonder aloud why we won't see more extensive strikes.